Liquid crystals currently are used in a wide variety of devices, including optical devices such as visual displays. Such devices usually require relatively low power and have satisfactory response time, provide reasonable contrast, and are relatively economical. The property of liquid crystals enabling use, for example, in visual displays, is the ability of liquid crystals to transmit light on one hand, and to scatter light and/or to absorb light, on the other, in dependence on whether the liquid crystals are in a relatively free, i.e. deenergized or field off state or in a strictly aligned, i.e. an energized or field on state, e.g. with an electric field applied across the liquid crystal material. An example of electrically responsive liquid crystal material and use thereof is provided in U.S. Pat. No. 3,322,485.
Certain liquid crystal material is responsive to temperature, changing optical characteristics in response to temperature of the liquid crystal material.
The invention of the present application is disclosed hereinafter particularly with reference to use of liquid crystal material that is particularly responsive to electric field.
Currently there are three categories of liquid crystal materials, namely cholesteric, nematic and smectic types. The invention of the present application relates in the preferred embodiment described below to use of nematic liquid crystal material or to a combination of nematic and some cholesteric type. However, various principles of the invention may be employed with various one or ones of the known types of liquid crystal material or combinations thereof. The various characteristics of the cholesteric, nematic and smectic types of liquid crystal material are described in the prior art. One known characteristic of liquid crystal material is that of reversibility; in particular it is noted here that nematic liquid crystal material is known to be reversibile, but cholosteric material is not reversible.
To enhance contrast and possibly other properties of liquid crystal material, pleochroic dyes have been mixed with the liquid crystal material to form a solution therewith. The molecules of pleochroic material generally align with the molecules of the liquid crystal material. Therefore, such pleochroic dyes will tend to function optically in a manner similar to that of the liquid crystal material in response to a changing parameter, such as application or non-application of an electric field. Examples of pleochroic dyes with liquid crystal material are described in U.S. Pat. Nos. 3,499,702 and 3,551,026.
An important characteristic of liquid crystal material is anisotropy. Anisotropic material has different physical properties in different directions. For example, liquid crystals may be optically anisotropic such that when the molecules are randomly oriented they permit light transmission and, when they are aligned due to application of an electric field thereacross, light may be transmitted through the liquid crystal material.
Liquid crystal material also has electrical anisotropy. For example, the dielectric constant for nematic liquid crystal material may be one value when the liquid crystal molecules are parallel to the electric field and may have a different value when the liquid crystal molecules are aligned perpendicular to an electric field. Since such dielectric value is a function of alignment, for example, reference to the same as a "dielectric coefficient" may be more apt than the usual "dielectric constant" label. Similar properties are true for other types of liquid crystals.
Some brief discussion of encapsulation of cholesteric liquid crystal material is presented in U.S. Pat. Nos. 3,720,623, 3,341,466, and 2,800,457, the latter two patents being referred to in the former patent.
In the past devices using liquid crystals, such as visual display devices or other devices, have been of relatively small size. Large size devices using liquid crystals, such as, for example, a billboard display or a sign have not been able to be made satisfactorily for a number of reasons. One reason is the fluidity of the liquid crystals, (the liquid crystal material may tend to flow creating areas of the display that have different thicknesses). As a result, the optical characteristics of the display may lack uniformity, have varying contrast characteristics at different portions of the display, etc; the thickness variations cause variations or gradations in optical properties of the liquid crystal device. Moreover, the varying thickness of the liquid crystal material will cause corresponding variations in the electrical properties of the liquid crystal material, such as capacitance and impedance. further reducing uniformity of a large size liquid crystal device. The varying electrical properties of the liquid crystal material, then, also may cause a corresponding variation in the effective electric field applied across the liquid crystal material and/or in response to a constant electric field would respond differently at areas of the liquid crystal that are of different thickness.
A pleochroic display, i.e. one in which pleocroic dye and liquid crystal material are in solution, has the advantage of not requiring a polarizer. However, such a pleochroic device has a disadvantage of relatively low contrast when used directly in the nematic form. It was discovered in the past, though, that a cholesteric liquid crystal material could be added to the nematic one together with the dye to improve the contrast ratio. See White et al. article "Journal of Applied Physics," Volume 45, No. 11, November 1974, at pages 4718-4723, for example. The cholesteric material would tend not to return to its original zero field form when the electric field is removed.
Another problem encountered with pleochroic dye included in solution with liquid crystal material, regardless of the particular type of liquid crystal material, is that the absorbtion of the dye is not zero in the field-on condition; rather such absorbtion in the field-on condition follows a so-called ordering parameter, which relates to or is a function of the relative alignment of the dyes. The optical transmission characteristic of liquid crystal material is an exponential function of thickness of the liquid crystal material; specifically, the on state or field-on or energized state of the liquid crystal material is an exponential function of thickness of the liquid crystal material, and the absorbing state or off state also is a different exponential function of thickness.
To overcome those problems described in the two immediately preceding paragraphs, the liquid crystal material should have an optimum uniform thickness. (As used herein liquid crystal material means tthe liquid crystals themselves and, depending on context, the pleochroic dye in solution therewith.) There also should be an optimum spacing of the electrodes by which the electric field is applied to the liquid crystal material. To maintain such optimum thickness and spacing, rather close tolerances must be maintained. To maintain close tolerances, there is a limit as to the size of the device using such liquid crystals, for it is quite difficult to maintain close tolerances over large surface areas, for example.